Optical fiber coupling componet

ABSTRACT

An optical fiber coupling part capable of reducing coupling loss while maintaining a large operating distance, and having a good module assembling property. AT least one GRIN lens having numerical aperture NA that is larger than numerical aperture NAs of a light-emitting source (such as a semiconductor laser) is fusion-spliced with one end of the optical fiber. All lights emitted from the light-emitting source can enter the GRIN lens, and the loss of the light can thereby be reduced. In addition, a second GRIN lens having numerical aperture NA 2  is fusion-spliced with one end of the optical fiber having numerical aperture NA f , and further a first GRIN lens having numerical aperture NA 1 , which is larger than numerical aperture NA 2 , is fusion-spliced with the other end of the second GRIN lens. Thereby, the light emitted from the light-emitting source can efficiently enter the optical fiber, and loss of the light can thereby be reduced. In this case, the formula expressed by NA f ≦NA 2 &lt;NA s ≦NA 1  is desirable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber coupling part forcoupling a light emitting source such as a semiconductor laser used foroptical communication and an optical fiber with high couplingefficiency.

2. Description of the Related Art

A technique for coupling a semiconductor laser and an optical fiber withhigh coupling efficiency is one of the most important techniques inoptical communication. For example, conventional methods of coupling thesemiconductor laser and the optical fiber include a method using a lenssuch as a spherical lens and an aspherical lens, or a method using a tipball fiber whose tip part is spherical (see U.S. Pat. No. 3,910,677).The method using the lens involves problems in that mutual alignment ofthe optical axes of the semiconductor laser, the lens, and the opticalfiber is complicated, enlarging the entire body of the coupling systemat the same time, thereby increasing the manufacturing cost, whilerelatively high coupling efficiency is obtained. In addition, since thedimension of the lens is large and a disposition space is therebylargely occupied, this method can not be used for coupling asemiconductor laser array and an optical fiber array in which aplurality of semiconductor lasers and a plurality of optical fibers arearranged at short intervals. Meanwhile, since formed in small size, themethod using the tip ball fiber is capable of coupling the semiconductorarray and the optical fiber array. The aforementioned optical fiber isintegrally formed with a hemispherical lens part at the tip of a singlemode optical fiber. Meanwhile, when making the tip ball optical fiber, aproblem is that conventionally the tip part of the fiber is polishedaround, and therefore mass-productivity is deteriorated and it takessignificant labor hours to produce. Another problem is that, since thetip of the optical fiber is spherical, coupling efficiency isdeteriorated due to spherical aberrations. Specifically, light beamemitted from a laser end face reaches the end face of the single modeoptical fiber at different positions and at different angles, dependingon the exiting angle of the outgoing light. Therefore, some of the lightbeams deviate from the core, or even when it reaches the core, anincident angle to the core is equal to or larger than a critical angle,and therefore the light is not propagated through the single opticalfiber to deteriorate coupling loss. For example, when a standardsemiconductor laser is used, the coupling loss is approximately 6 dB.

In order to solve the above-described problems, a columnar graded indexlens (called “GRIN lens” hereafter) facilitating optical axis alignmentis used. The GRIN lens is the lens using medium materials not uniform inrefractive index (refractive index becomes larger toward the center ofthe lens), and functions as a lens whose refractive index iscontinuously changing. Refractive index distribution n (r) of the GRINlens in a radius direction is expressed by the following formula:n(r)=n ₀(1−(½)(gr)²)  (FIG. 1)where, n (r) is a constant expressing the refractive index at distance rfrom the center, n₀ is a constant expressing the refractive index at thecenter part, and g is a constant expressing a light-condensingperformance of the GRIN lens. The above-described lens has relativelysmall spherical aberrations. However, the critical angle of theconventionally existing GRIN lens is made small, set to be 20° or less.Thus, such a lens can not sufficiently take in lights emitted from thesemiconductor laser for optical communication, whose standard radiationfull angle at half maximum θ is approximately 25°, thereby increasingthe coupling loss. Therefore, a ball lens and the GRIN lens arefrequently used in combination. However, in this case, it is difficultto align the optical axis, thereby increasing the assembly cost.Moreover, it is designed so that the tip of the GRIN lens is cut inspherical shape to increase NA in appearance (to improve thelight-condensing performance). However, the problems are thatmass-productivity is deteriorated and it takes significant labor toproduce, thereby increasing the manufacturing cost. Further, the GRINlens is conventionally made of a multi-component glass, and itssoftening point is about 500 to 600° C. Therefore, such a GRIN lens cannot be fusion-spliced with the optical fiber, which is mainly composedof quartz glass. Thus, an optical adhesive is used, thereby posingproblems in that it is difficult to align the optical axis, and anoptical characteristic is deteriorated by a change in the quality of theadhesive caused by temperature-raise, when the adhesive absorbs thelight and high intensity light thereby enters.

In order to solve such a problem of connection deterioration, astructure using GI (Graded-Index) optical fiber as a lens has beenproposed (see U.S. Pat. Nos. 4,701,011 and 5,384,874).

The GI optical fiber is the optical fiber in which the refractive indexof a core part changes in a radial direction. Since the GI optical fiberis made of the same quartz as the optical fiber, the GI optical fibercan be fusion spliced with the optical fiber. Therefore, it can beexpected that the GI optical fiber will have high durability againstlight of high intensity. However, in this case, the critical angle ofthe GI optical fiber is made small, set to be 20° C. or less,(light-condensing performance is small), and therefore it is difficultto sufficiently take in the light emitted from the semiconductor lensfor optical communication, whose standard radiation full angle at halfmaximum θ is approximately 25°, and therefore the coupling loss islarge, and handling property is low when actually assembled as a lens.

In order to solve the above-described problems, it is desired to developa GRIN lens having light-condensing performance (high numericalaperture) which is high enough to sufficiently cover the emission angleof a semiconductor laser. Particularly, a standard radiation full angleat half maximum of a semiconductor laser is equal to 25° or larger.Therefore, in order to sufficiently guide the light of the semiconductorlaser to the GRIN lens, it is necessary to develop a GRIN lens having acritical angle of at least 25° or larger. The critical angle correspondsto a maximum angle formed with an axis which allows the light to enterthe optical fiber and the GRIN lens, when the light enters the opticalfiber and the GRIN lens at an angle relative to the axis. Usually, asine function of the critical angle is referred to as numerical aperture(referred to as “NA” hereafter). When the radiation full angle at halfmaximum of the semiconductor laser is 25°, the numerical aperture NAs is0.43. Therefore, when the GRIN lens having NA which is equal to 0.43 orlarger is used, all the lights of the semiconductor laser can enter thelens. Thus, such a GRIN lens is required. In addition, in order tofacilitate the optical alignment of the axes of the semiconductor laser,the GRIN lens, and the optical fiber, a coefficient of thermal expansionof the GRIN lens needs to be set at 15×10⁻⁷K⁻¹ or less, while thecoefficient of thermal expansion of quartz is set at 5×10⁻⁷K⁻¹. Theabove-described fusion splicing is a required technique for improvingproductivity, and by the fusion-splicing, the light reflected from aboundary surface between the optical fiber and the lens and returned tothe semiconductor laser is reduced, to solve the problem that theoptical characteristic is deteriorated by a change in the quality of theadhesive caused by temperature-raise, when the adhesive absorbs thelight, and high intensity light enters. In addition, if the opticalfiber and the GRIN lens having approximately the same sectional shapeare fusion-spliced under flame using an oxyhydrogen burner, etc., due toa self-aligning effect (effect that center axes of both of the opticalfiber and the GRIN lens are naturally coincident with each other by thesurface tension of fused glass), the center axes of the optical fiberand the lens are coincident with each other without accurate axisaligning which has been a long-pending problem, thereby obtaining alarge advantage in that the assembling property is significantlyimproved.

A method of efficiently condensing the light of the semiconductor laserby using the GRIN lens having high light-condensing performance asdescribed above, includes the method of directly fusion-splicing theGRIN lens having high NA with the tip of the optical fiber. However, inthis case, the coupling loss of about 3-4 dB must be expected. Thereason is that, although the light radiated from the end face of thesemiconductor laser is condensed on the end face of the single modeoptical fiber by the light-condensing effect of the GRIN lens havinghigh NA, a part of the light having a large emission angle reaches anangle that is larger than the critical angle of the optical fiber.Particularly, the problem is that, when the critical angle of thesemiconductor laser (sine function of this critical angle=numericalaperture called NAs) is larger than the critical angle of the opticalfiber (sine function of this critical angle=numerical aperture calledNA_(f)), the light deviates from the core of the optical fiber dependingon the emission angle of the light beam, or even if the light reachesthe core, the incident angle to the core is equal to the critical angleor larger, thereby failing with regard to entering the single modeoptical fiber and deteriorating the coupling loss.

In order to solve the above-described problem, an optical fiber withlens has been proposed (see Japanese Patent Laid Open No. 8-292341). Inthe optical fiber with lens, one end of the single mode optical fiberhaving a core and a clad and the other end of the coreless optical fiberare connected by a 2nd square type optical fiber (corresponding to GRINlens). The 2nd square type optical fiber has a 2nd square typerefractive index distribution of length of nearly ¼ as long as a zigzagcycle of a light beam propagated through the lens or the length of anodd number times of the length of ¼ of the zig-zag cycle. The opticalfiber with lens is formed by connecting the 2nd square type opticalfiber (corresponding to GRIN lens) having 2nd square type refractiveindex distribution of length of nearly ¼ as long as a zigzag cycle of alight beam propagated through the lens or the length of an odd numbertimes of the length of the ¼ of the zig-zag cycle, to the single modeoptical fiber having the core and the clad. Here, the 2nd square typeoptical fiber has the core and the clad, and the tip is formed insemi-spherical shape. By using the above-described optical fiber, thecoupling loss is reduced to approximately 4 dB when coupled to thesemiconductor laser, which is not enough to satisfy the coupling loss (3dB or less) required practically. Generally, the smaller the couplingloss of the semiconductor laser and the optical fiber, the higher theperformance of an optical communication system becomes, thereby alsofacilitating system construction. In addition, the tip is formed insemi-spherical shape with low yield ratio at a high cost. In order tonot significantly lower the coupling efficiency of the lens havingsemi-spherical tip and the semiconductor laser, distance between thesemi-spherical lens and the semiconductor laser, in other words,operating distance must be approximately 10 μm. Therefore, adisadvantage is that when constructing a coupling system of coupling theoptical fiber with semi-spherical lens and the semiconductor laser, thesemiconductor laser and the semi-spherical lens collide with each other,resulting in being unusable.

However, a conventional technique relating to the optical fiber withlens can not simultaneously satisfy requirements such as realizing morereduced coupling loss, further facilitating aligning of the optical axesof a semiconductor laser, a lens, and an optical fiber, whilemaintaining a long operating distance. In view of the above-describedproblems, the present invention is provided, and an object of thepresent invention is to provide the optical fiber with a GRIN lens and alaser module capable of reducing the coupling loss while maintaining along operating distance and having a good module assembling property.

SUMMARY OF THE INVENTION

(Structure 1)

A first structure of the present invention provides an optical fibercoupling part, wherein at least one GRIN lens having a numericalaperture NA larger than the numerical aperture NAs of a light emittingsource (such as semiconductor laser) is fusion-spliced with one end ofthe optical fiber.

The optical fiber part of the present invention functions to efficientlysend light emitted from a light emitting source to the optical fiber foroptical communication, by arranging the end part of the GRIN lensopposed to the light emitting source, and connecting the optical fiberfor optical communication to the other end. The GRIN lens, as describedabove, is the lens using medium materials that are not uniform inrefractive index (refractive index becomes larger toward the center ofthe lens), and functioning as a lens having continuously changingrefractive index. FIG. 1 is an explanatory view of the GRIN lens,illustrating a refractive index distribution in a radius direction inthe left side, and illustrating its perspective view in the right side.As shown in FIG. 1, the GRIN lens has a 2nd square type refractive indexdistribution. The numerical aperture NAs of the light emitting source(such as semiconductor laser) is a sine function of a critical angle ofa radiation full angle at half maximum, and the numerical aperture NA ofthe GRIN lens is a sine function of a critical angle of the GRIN lens.The light emitting source having larger numerical aperture spreads thelight in a larger area, and the GRIN lens having larger numericalaperture has a higher light-condensing performance.

Conventionally, there exists no GRIN lens having numerical aperture NAlarger than the numerical aperture NAs of the semiconductor laser andcapable of being fusion-spliced with the optical fiber. However, such aGRIN lens can be manufactured by methods and embodiments as will bedescribed hereunder. Since the numerical aperture of the GRIN lens islarger than the numerical aperture of the light emitting source, all ofthe light emitted from the light-emitting source can enter the GRINlens, thereby reducing the loss of the light.

(Structure 2)

A second structure of the present invention provides the optical fibercoupling part according to the first structure, wherein a numericalaperture NA is equal to 0.43 or larger. As described above, thenumerical aperture of a general light emitting source (semiconductorlaser) is 0.43, and therefore by setting the numerical aperture of theGRIN lens at 0.43 or more, the numerical aperture larger than thegeneral light emitting source is obtained.

(Structure 3)

A third structure of the present invention provides the optical fibercoupling part according to either of the first structure or the secondstructure, wherein a coefficient of thermal expansion of the GRIN lensis set at 15×10⁻⁷K⁻¹ or less, and the optical fiber coupling part isprovided by a sol-gel method. By setting the coefficient of thermalexpansion of the GRIN lens at 15×10⁻⁷K⁻¹ or less, the GRIN lens can befusion-spliced with the optical fiber, which is a quartz glass withoutdefect. Thus, productivity is excellent (for example, optical axisaligning is eliminated and high yield ratio is thereby obtained), andproblems such as change in the quality of a connection part between theGRIN lens and the optical fiber, and the loss of light can be solved.The GRIN lens having such a thermal expansion ratio can be manufacturedby the sol-gel method. The sol-gel method will be explained in detailhereunder.

It is extremely difficult to form a GRIN lens having the samecoefficient of thermal expansion as that of quartz glass, by aconventional ion exchanging method or chemical vapor deposition. TheGRIN lens made by the ion exchanging method is multi-component glasscontaining an alkaline component having an extremely large coefficientof thermal expansion, thereby posing a problem such as reliability overheat resistance. Also, in the chemical vapor deposition, 0.38 NA (forexample, see the document; P.B.O' et al.: ELECTION. Lett.,13(1977)170-171) is obtained. However, if an amount of an additive (suchas GeO₂, P₂O₅) is increased for the purpose of obtaining NA larger than0.38, the coefficient of thermal expansion becomes larger, therebyeasily breaking a preform. As described above, the problem is poormatching of the coefficient of thermal expansion with the preform.

Only one method capable of solving such a problem is a sol-gel methodbased on a low temperature synthetic method. In the sol-gel method, anacid or a base abduct salt is added as a solvent into an alcoholicsolution mainly composed of alkoxide (Si (OR)₄ (R: alkyl group)) ofsilicon, so that the alcoholic solution is hydrolyzed by the acid or thebase abduct salt to form a sol. When a multi-component glass is formed,a metal component is further added, to subject the sol to furtherpolycondensation, thereby advancing crosslinking reaction to form a wetgel. Then, by drying the wet gel thus obtained and by baking the driedgel after removing the solvent in the gel, a dense glass is formed. Whenthe GRIN lens is formed by using the sol-gel method, concentrationdistribution needs to be formed in the metal component. Since therefractive index becomes larger in a part thick in concentration of themetal component, the concentration of the center part of the GRIN lensis made thick in a profile of making the concentration thin towardoutside. A raw material of the metal component includes a method using ametal alkoxide or metal salt, and further includes a molecule staffingmethod and the like.

In order to examine the metal component to be added into the GRIN lensof the present invention for examining the metal component for enlargingthe refractive index, the refractive index of a binary quartz glass isexpected by using the calculating formula of the well-knownLorentz-Lorenz. Then, candidates of the metallic additive components ofthe GRIN lens include the group of SiO₂—Bi₂O₃, —In₂0₃, —Y₂O₃, —La₂0₃,—Ga₃O₂, —Sb₃O₂, —Gd₃O₂, —Nb₂O₅, —SnO₂, —Ta₂O₅, —TiO₂, and —ZrO₂. Amongthe above components, it was clarified that composition containing Bi,In, Y, and La was a slightly alkoxide soluble solid, allowing no gel tobe formed. Also, the composition containing Gd and Ga, has onlynumerical aperture (NA) 0.3 or less in a region with less additive (anamount of the additive to be added to Si is 20 mol % or less). Further,the Nb and Sn added glass has a crystalline substance recognized thereinand has a large coefficient of thermal expansion, and is thereforeunsuitable for the GRIN lens. Moreover, the Sb added glass and the Zradded glass exhibit instability in the process. Specifically, in the Sbadded glass, an added element Sb is evaporated when the gel is baked,and in the Zr added glass, although small in quantity, precipitation isformed in methanol used as the solvent in the process of forming thegel.

From the examination result thus described above, SiO₂—Sb₂O₃,SiO₂—Ta₂O₅, SiO₂—Ti₂O₃, and SiO₂—ZrO₂ based quartz glass, furtherpreferably, taking stability of the process into consideration,SiO₂—Ta₂O₅, SiO₂—Ti₂O₃ based quartz glass are preferable. When Ta: 10mol % and Ti: 12 mol % are respectively added by the sol-gel method, itwas found that the GRIN lens having high NA and the coefficient ofthermal expansion approximating that of the quartz glass can be formed.

(Structure 4)

A fourth structure of the present invention provides an optical fibercoupling part, wherein a second GRIN lens having numerical aperture NA₂is fusion-spliced with one end of the optical fiber having numericalaperture NA_(f), and further a first GRIN lens having numerical aperturelarger than NA₂ is fusion-spliced with the other end of the second GRINlens.

In the optical fiber coupling part of the present invention, byarranging the end part of the first GRIN lens side so as to be opposedto a light-emitting source, and by connecting the optical fiber foroptical communication to the other end, light emitted from thelight-emitting source is efficiently sent to the optical fiber foroptical communication. The light emitted from the light-emitting sourcesequentially passes through the first GRIN lens and the second GRINlens, to enter the optical fiber. However, since the numerical apertureNA₁ of the first GRIN lens is larger than the numerical aperture NA₂ ofthe second GRIN lens, the GRIN lens having the larger numerical aperture(preferably a numerical aperture larger than the numerical aperture NAsof the light-emitting source) is adopted, and the light emitted from thelight-emitting source can thereby efficiently enter the first GRIN lens.Also, since the numerical aperture NA2 of the second GRIN lens issmaller than NA1, the GRIN lens having sufficiently a small numericalaperture can be selected, and a critical angle of the light that reachesthe optical fiber from the second GRIN lens can be made small (when thenumerical aperture is small, zigzag cycle of the light moving throughthe GRIN lens becomes long, and the critical angle of the outgoing lightfrom the GRIN lens becomes accordingly small). Therefore, the lightefficiently enters the optical fiber from the second GRIN lens.

(Structure 5)

A fifth structure of the present invention provides the optical fibercoupling part according to the fourth structure, wherein the numericalaperture of the optical fiber (NA_(f)), the numerical aperture of thefirst GRIN lens (NA₁), the numerical aperture of the second GRIN lens(NA₂), and the numerical aperture of the light-emitting source (NA_(s))are selected so as to satisfy the formula expressed by:NA _(f) ≦NA ₂ <NA _(s) ≦NA ₁.

Since the formula expressed by NA_(s)≦NA₁ is established, all of thelight emitted from the light-emitting source enter the first GRIN lens,to thereby eliminate the loss of light. Also, since the formulaexpressed by NA_(f)≦NA₂<NA_(s) is established, the critical angle of thelight that reaches the optical fiber from the second GRIN lens becomessmall, and therefore the light efficiently enters the optical fiber fromthe second GRIN lens. Accordingly, the light emitted from thelight-source efficiently enters the optical fiber in total. Note thatusually, equations expressed by NA_(f)=0.15, and NA_(s)=0.43 areestablished.

(Structure 6)

A sixth structure of the present invention provides the optical fibercoupling part according to either of the fourth structure or fifthstructure, wherein the numerical aperture NA1 of the first GRIN lens is0.43 or more. As described above, since the numerical aperture of ageneral light-emitting source (semiconductor laser) is 0.43, byspecifying the numerical aperture of the first GRIN lens as 0.43 ormore, the numerical aperture that is larger than the generallight-emitting source is obtained.

(Structure 7)

A seventh structure of the present invention provides the optical fibercoupling part according to any one of the fourth structure to the sixthstructure, wherein when the refractive index of the center of the glassis set at n₀, radius of the lens 1 is set at d₁, and distance betweenthe GRIN lens and the light-emitting source is set at L, the length Z₁of the first GRIN lens satisfies the formula expressed by:Z 1=(n _(o) *d ₁ /NA ₁)arctan(D ₁ /NA ₁ *L)).

When the expression Z₁=(n_(o)*d₁/NA₁) arctan (D₁/NA₁*L)) is established,the light that enters the first GRIN lens becomes parallel light beamsin the final end, and the parallel light beams efficiently enter thesecond GRIN lens. Also, along with the large numerical aperture of thefirst GRIN lens, the distance between the GRIN lens and thelight-emitting source is made long, and the assembling property canthereby be improved.

(Structure 8)

An eighth structure of the present invention provides the optical fibercoupling part according to the seventh structure, wherein since thelength Z₂ of the second GRIN lens has the length of nearly ¼ as long asa zigzag cycle of a light beam propagated through the lens or the lengthof an odd number times of the length of ¼ of the zig-zag cycle.

In the seventh structure, parallel light beams enter the second GRINlens from the first GRIN lens. Since length Z₂ of the second GRIN lensis nearly ¼ as long as a zigzag cycle of a light beam propagated throughthe lens or the length of an odd number times of length of ¼ of thezig-zag cycle, the parallel light that enter is condensed in a centeraxis of an optical fiber 4 in the final end. At this time, since thelight-condensing property of the second GRIN lens is smaller than thatof the first GRIN lens, the lights are condensed at a loose angle, andtherefore the lights efficiently enter the optical fiber.

(Structure 9)

A ninth structure of the present invention provides the optical fibercoupling part according to any one of the fourth structure to the eighthstructure, wherein the coefficient of thermal expansion of the first andsecond GRIN lenses is expressed by 15×10⁻⁷K⁻¹ or less and at least thefirst GRIN lens is formed by a sol-gel method. By setting thecoefficient of thermal expansion at 15×10-⁷K⁻¹ or less, the first andsecond GRIN lenses can be fusion-spliced with each other, and the secondGRIN lens and the optical fiber, which is a quartz glass, can befusion-spliced with each other without defects. Thus, productivity isexcellent (optical axis aligning is eliminated and high yield ratio isthereby obtained, etc), and problems such as a change in quality of aconnection part between the GRIN lens and the optical fiber, and theloss of light can be solved. The first GRIN lens having such acoefficient of thermal expansion and having a large numerical aperturecan be manufactured by the sol-gel method. The second GRIN lens having asmall numerical aperture can also be manufactured by the conventionalwell-known method.

(Structure 10)

A tenth structure of the present invention provides the optical fibercoupling part according to any one of the first structure to ninthstructure, wherein the optical fiber is a single mode optical fiber. Inthe coupling part of the present invention, a most general single modeoptical fiber can be used as an optical fiber. The single mode opticalfiber usually comprises a core of the center part having relativelylarge refractive index, and clads having relatively small refractiveindex around the core, and the diameter of the core is about 10 μm, andthe diameter of the clad (diameter of the fiber) is about 125 μm.

According to the optical fiber coupling part with GRIN lens, a couplingprofile can be made to have substantially the same thickness as that ofthe optical fiber, thereby providing a small coupling system in total.Therefore, the semiconductor array and the optical fiber array can becoupled to each other, so that a plurality of semiconductor lasers and aplurality of optical fibers are arranged at short intervals. Theoperating distance (distance between the lens and the light-emittingsource) can be made large, thereby facilitating the assembly of thecoupling system without damaging the lens, and the coupling loss can besignificantly reduced. In addition, all the optical fiber coupling partswith GRIN lens of the present invention are optical fiber types, and canbe formed by using the existing fusion splicing technique as it is,thereby having a significant effect that mass production is realizedwith easy manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a GRIN lens.

FIG. 2 is an explanatory view of an optical fiber coupling part of anembodiment.

FIG. 3 is an explanatory view of a forming process of the optical fibercoupling part of an embodiment.

FIG. 4 is an explanatory view of the forming process of the GRIN lens.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Next, embodiments of the present invention will be explained based onFIG. 2. When specifying the distance between a semiconductor laser 3 anda first GRIN lens 1 as L, light emitted from the semiconductor laser 1at radiation full angle at half maximum θ corresponding to NA_(s) isreceived by the first GRIN lens 1 so as to be propagated through thefirst GRIN lens 1. At this time, the refractive index of the center partof the first GRIN lens 1 is set at N_(o), and the radius is set at d₁,and when solving a beam equation in the first GRIN lens 1, by adjustingthe length Z₁ of the first GRIN lens 1 like:Z ₁=(n _(o) *d ₁ /NA ₁)ARCTAN(D ₁/(NA ₁ *L)  (1),all the light beams propagated through the length of Z₁ within theradiation full angle at half maximum θ become parallel to the opticalaxis of the optical fiber. However, in order to prevent the conditionthat the light emitted from the semiconductor laser, reaches the sideface of the GRIN lens 1 of radius d₁ and is kicked by the side face,under formula expressed by:NA₁≧NA_(s)  (2),formula expressed by:d ₁ ≧L/(1/NA _(s)−1/NA ₁)^(1/2)  (3)may be approximately satisfied. As clarified by the formulas (1), (2),(3), particularly when formula, L/d₁<<1 is satisfied, formula expressedby Z₁˜(n_(o)*d₁/NA₁)*(π/2) is obtained, and therefore under thecondition of (2), even if radius d is set to be any value, the lightthat enters the GRIN lens 1 is not kicked by the side wall of the GRINlens 1.

Next, the parallel light beams thus obtained are caused to enter thesecond GRIN lens 2 (called NA₂) having the same or slightly larger NA asthe optical fiber 4 having NA_(f), which is expressed by:NA_(f)≦NA₂  (4).At this time, in order to prevent all the parallel light beams of thefirst GRIN lens 1 from being kicked by the side wall of the second GRINlens so as to be taken in the second GRIN lens having radius d₂ andnumerical aperture NA₂, when solving beam equations in the first GRINlens and the second GRIN lens, radius d1 and radius d₂ may be specifiedso as to satisfy the formula expressed by:8NA _(f) /NA ₂)*(D ₂ /D ₁)≧NA _(s)((L/d ₁)² NA ₁+1/NA ₁)  (5).Particularly, as clarified from formula (5), when formula L/d₁<<1 isestablished, radius d₁ and radius d₂ may be determined so as to satisfythe formula expressed by:d ₂ /d ₁≧(NA ₂ /NA _(f))*(NA _(s) /NA ₁)  (6).When length Z₂ of the second GRIN lens 2 is set to be nearly ¼ as longas a zigzag cycle of a light beam propagated through the second GRINlens 2 as expressed by:Z ₂=(n _(o) *d ₂ /NA ₂)*(π/2)  (7),or set to be the length odd number times of it, the lights are condensedin the center axis of the optical fiber 4 at a loose angle by the secondGRIN lens 2 having a smaller light-condensing property compared with thelight-condensing property of the first GRIN lens 1. Particularly notableis that the angle formed by the light condensed in the center axis ofthe optical fiber 4 and the center axis is the same or slightly smallerthan the critical angle of the optical fiber due to the smalllight-condensing performance of the second GRIN lens. Therefore, almostall of the light beams are received by the optical fiber, and thecoupling efficiency is thereby remarkably improved.

Of course, since the first GRIN lens 1, the second GRIN lens 2, andfurther the optical fiber 4 are fusion-spliced, the reflection loss oneach splicing surface reaches substantially zero. In short, in order toremarkably improve the coupling efficiency particularly under L/d₁<<1,preferably the first and second GRIN lenses 1 and 2 satisfying theformulas expressed by:NA _(f) ≦NA ₂ <NA _(s) ≦NA ₁  (8)d ₂ /d ₁≧(NA ₂ /NA _(f))*(NA _(s) /NA ₁)  (9)are fusion-spliced with tip of the optical fiber 3.

In view of the structure described above, an explanation will beprovided of a case of the formula satisfying L/d₁<<1. First, theoperating distance L (for example 30 μm), allowing the optical fiberwith GRIN lens to easily approach the semiconductor laser, isdetermined. Next, the second GRIN lens 2 having radius d₂, and numericalaperture NA₂ satisfying the formula (8) is selected, and the length isset to be the length (formula (7)), which is nearly ¼ as long as thezigzag cycle of a light beam propagated through the second GRIN lens 2,or the length Z₂ which is an odd number times of it. Next, by usingformula (9), radius d₁ of the first GRIN lens 1 is obtained. Usually,the radius is set to be d_(a)=d₂. The value thus obtained is substitutedfor formula (1), and length Z₁ of the first GRIN lens 1 is specified byusing the operating distance. Thus, by specifying distance L between thefirst GRIN lens and the semiconductor laser, mutual contact or collisionis eliminated when assembling. In addition, as clarified by formula (1),length Z₁ is not uniquely determined in property of an inversetrigonometric function, but is obtained by multiplying π times. However,although length Z₁ may be determined in consideration of the each ofmachining of the GRIN lens 1, preferably it is obtained by multiplyingπ-times.

Incidentally, the optical fiber 4 is explained as a single mode opticalfiber. However, the optical fiber 4 is not limited to the single modeoptical fiber, but may be a multi-mode optical fiber, provided thatformula (8) is satisfied. In addition, in a high-output multi-mode LD,the ratio of a light-emitting region in a parallel direction to alight-emitting region in a vertical direction ranges from several tens:1to several hundred:1, and in addition, vertically spreading angle θv andhorizontally spreading angle θp of a laser beam are extremely differentin relation to each other (θv>>θp). Therefore, in the above-describedrotational symmetric optical system, it is difficult to cause LD laserbeam to be efficiently guided into the optical fiber having a lightincident aperture that has good symmetricalness (such as circular form).As a counter measure, a planar plate-like GRIN lens (called NA₁) havingthe same or a slightly larger numerical aperture NA as the NA (calledNA_(s)) corresponding to the vertically spreading angle θv is insertedinstead of the cylindrical first GRIN lens 1, and only verticallyspreading angle θv may be adjusted by the first planar plate-like GRINlens.

In addition, usually, the light emitted from the semiconductor laser hasan elliptical shape. However, the first GRIN lens 1 is not limited to acylindrical shape, but may have elliptical-shaped refractive indexdistribution. In any case, irrespective of the shape, by combination ofthe GRIN lenses satisfying formula (2), high coupling efficiency can beobtained.

Next, a forming method of the embodiments of the present invention willbe explained based on FIGS. 2 and 3. As shown in FIG. 2, in asemiconductor laser module (coupling system), a semiconductor laser 3and the optical fiber 4 with first and second GRIN lenses are arrangedso as to be opposed to each other while maintaining the operatingdistance of about 30 μm therebetween. In the semiconductor laser 3, forexample, peak oscillation wavelength is set at 1330 m, operating currentis set at 16 m A, operating voltage is set at 1.0 V, horizontallyradiation full angle at half maximum is set at 20°, and verticallyradiation full angle at half maximum is set at 25°. In the optical fiberwith GRIN lens, the second GRIN lens 2 and the first GRIN lens 1 areconnected in this order to one end of the single mode fiber 4 havingcores and clads, and having a numerical aperture expressed byNa_(f)=0.15. Diameters of the first GRIN lens 1 and the second GRIN lens2 are set at the same diameter or a slightly larger diameter than thediameter of the optical fiber 4. In the example of FIG. 2, the diametersof the GRIN lenses 1 and 2 are equally set at 150 μm, respectively, andnumerical apertures NA₁ and NA₂ are set at 0.5 and 0.16, respectively.The second GRIN lens 2 has a length that is nearly ¼ as long as a zigzagcycle of a light beam propagated through the lens, and set atapproximately 860 μm by formula (7). Meanwhile, the length of the firstGRIN lens is obtained by the above described formula (1), and set atapproximately 990 μm by multiplying π-times, here.

The optical fiber with GRIN lens thus structured, is manufactured aswill be described below. First, as shown in FIG. 3B, the second GRINlens 2 having 2nd square type refractive index distribution havingnumerical aperture satisfying NA₂=0.16 and diameter 150 μm isfusion-spliced with one end of the single mode optical fiber 4 having125 μm diameter and numerical aperture satisfying NA_(f)=0.15 by using afusion splicer. Thereafter, the second GRIN lens 2 and the single modeoptical fiber thus fusion-spliced is cut in a 860 μm length which isnearly ¼ as long as a zigzag cycle of a light beam propagated throughthe second GRIN lens (FIG. 3B). Next, as shown in FIG. 3(C) a stringmaterial of the first GRIN lens 1 of proper length having numericalaperture satisfying NA₁=0.5 and the 2nd square type refractive indexdistribution of 150 μm diameter which are different from those of thesecond GRIN lens, is fusion-spliced with the second GRIN lens 2.Thereafter, by cutting and grinding the first GRIN lens 1, the length ofthe first GRIN lens 1 is made to be 990 μm long, and the optical fiberwith GRIN lens is thereby obtained (FIG. 3C).

By using the above-described semiconductor laser module, thesemiconductor laser was made to be opposed to the optical fiber withGRIN lens holding the distance of 30 μm therebetween, the semiconductorlaser having radiation characteristics of 1330 nm peak oscillationwavelength, 16 mA operating current, 1.0V operating voltage,horizontally radiation full angle at half maximum 20°, and verticallyradiation full angle at half maximum 25°, and the optical fiber havingnumerical aperture satisfying NA₁=0.5. Then, extremely high couplingefficiency with coupling loss of 1 dB or less was obtained, therebyverifying the superiority of the present invention.

Embodiment 1

2 normal hydrochloric acid 9.2 ml was added into a mixture of 75.5 mlsilicon tetramethoxide and 183.4 ml isopropanol, and after stirring for30 minutes, 9.8 ml titanium tetra-n-butoxide was added. Thereafter, 0.01normal ammonia water was added, and stirred, to obtain a wet gel. Afteraging the wet gel at a temperature of 50° C. for two days, the wet gelis immersed in 6 normal hydrochloric acid for two hours, so thattitanium on an outer periphery is eluted, and concentration distributionof titanium was imparted in the gel. After being immersed, the wet gelis dried at a temperature of 70° C., to obtain a dry gel having about 10mm diameter. The dry gel thus obtained was subjected totemperature-raise from room temperature to 800° C. at a rate of 150°C./hr in an oxygen atmosphere, and thereafter to 1250° C. at a rate of50° C./hr in a helium atmosphere, and then subjected to baking to obtaina transparent glass body. The refractive index distribution of thecolumnar glass body was measured, and as a result, the refractive indexdistribution of the preform of the second GRIN lens was obtained in theprofile of substantially square curved decrease expressed by equationNA=0.16 from the center toward the periphery.

Next, 2 normal hydrochloric acid 9.2 ml was added into the mixture of75.5 ml silicon tetramethoxide and 183.4 ml isopropanol, and afterstirring for 30 minutes, 30.8 ml titanium tetra-n-butoxide was added.Thereafter, 0.01 normal ammonia water was added and stirred, to obtainwet gel. After aging the wet gel at a temperature of 50° C. for twodays, the wet gel was immersed in 6 normal hydrochloric acid for twohours, and the concentration distribution of titanium was imparted inthe gel. After being immersed as described above, the gel was immersedin methanol, and hydrochloric acid wash in the gel was conducted.Thereafter, the gel was immersed in 6 normal hydrochloric acid for 20minutes, and the second concentration distribution was imparted, and inthe same way as the first time, the gel was immersed in the methanol andhydrochloric acid wash in the gel was conducted, and then dried. The gelthus dried was immersed in 6 normal hydrochloric acid for 8 minutes, toimpart a third concentration distribution. Then, the gel was immersed inmethanol in the same way as the first time, and after hydrochloric acidwash, the gel was dried to obtain a dry gel having about 10 mm diameter.The dry gel thus obtained was subjected to temperature-raise from roomtemperature to 350° C. at a rate of 10° C./hr, thereafter, to 1200° C.,and then subjected to baking to obtain a transparent glass body. Therefractive index distribution of the columnar glass body was measured,and as a result, it was clarified that the preform of the first GRINlens has the refractive index distribution in the profile ofsubstantially square curved decrease expressed by equation NA=0.5, fromthe center toward the periphery. Thus, by imparting concentrationdistribution multiple times in a state of wet gel, the GRIN lens havinglarge numerical aperture can be formed.

While these two preforms were inserted into an electric furnace of acarbon heater separately at a rate of 0.04 mm/s, the preforms wereformed into a GRIN lens-like optical fiber having 150 μm outer diameterby spinning, and the first GRIN lens-like optical fiber and the secondGRIN lens optical fiber were thereby formed. Then, the second GRINlens-like optical fiber thus formed was fusion-spliced with one end ofthe single mode optical fiber having numerical aperture of 0.15 by usinga discharge fusion splicer. Thereafter, the second GRIN lens-likeoptical fiber and the single mode optical fiber thus fusion-spliced wascut in a 990 μm length which is nearly ¼ as long as a zigzag cycle of alight beam propagated through the second GRIN lens. Next, the first GRINlens-like optical fiber having numerical aperture 0.5 different fromthat of the second GRIN lens was fusion-spliced with the second GRINlens by using the same discharge fusion splicer as used in the casedescribed above. Thereafter, the first GRIN lens-like optical fiber andthe second GRIN lens thus fusion-spliced was cut and ground so as to be860 μm length, and the optical fiber coupling part with the first andsecond GRIN lenses of a first embodiment was thereby obtained.

The optical fiber coupling part thus obtained was opposed to thesemiconductor laser having 1330 nm peak oscillation wavelength, 16 mAoperating current, 1.0V operating voltage, horizontally radiation fullangle at half maximum 20°, and vertically radiation full angle at halfmaximum 25°, holding operating distance of 30 μm therebetween, and inthis case, high coupling efficiency with coupling loss of 0.9 dB or lesswas obtained.

Embodiment 2

First, in the same process as the first embodiment, after forming thepreform of the second GRIN lens having numerical aperture satisfyingNA=0.16, the preform was formed into the second GRIN lens-like opticalfiber having 150 μm outer diameter by spinning in the electric furnaceof the carbon heater.

Next, after 2 normal hydrochloric acid 9.2 ml was added into a mixtureof 76.6 ml silicon tetramethoxide and 184.3 ml isopropanol, 50 mlsuperfine particulate silica was added to mix therewith, and the mixturethus obtained was stirred for 1 hour and partially hydrolyzed. Then, thesolution was divided equally into eight parts, and added with titaniumtetra-n-butoxide of concentration as shown in Table 1, to make eightkinds of sol from first layer to eighth layer with different titaniumcomponents at regular intervals. Thereafter, 0.01 normal ammonia waterwas added at regular intervals to adjust the sol.

First, the sol of the first layer was contained in a polypropylenevessel having a 50 mm inner diameter, and the sol thus contained wasrotated at a rate of 1100 revolutions/min, for 30 minutes, and acylindrical wet gel was formed on an inner wall of a cylindrical vessel21. Thereafter, in a similar process, sol liquid with different titaniumcomponents from second layer to eighth layer were sequentially containedin the vessel 21 to concentrically laminate a wet gel layer 22 withdifferent added amount of titanium of eight layers, on the inner wall ofthe vessel 21 (FIG. 4A). While the cylindrical wet gel thus made wasrotated, the wet gel was dried at 60° C. for one week to obtain a drygel. The dry gel was contracted and formed into a cylinder having a 26mm inner diameter, 13 mm outer diameter, and 0.04% or less ellipticity.The dry gel thus obtained was subjected to temperature-raise to 800° C.at a rate of 150° C./hr in an oxygen atmosphere, and thereafter, to1250° C. at a rate of 50° C./hr in a helium atmosphere, and thensubjected to baking to obtain a transparent glass body 23. Both ends ofthe cylindrical glass body 23 was fixed to a lathe turning machine, andwhen the glass body 23 was sequentially heated from an end part whilerotating, by using an oxyhydrogen burner 25 of about 2000° C., acylindrical GRIN lens preform 24 with a closed inner diameter wasobtained (FIG. 4C). TABLE 1 Titanium added amount First layer   0 mlSecond layer 1.1 ml Third layer 1.6 ml Fourth layer 2.1 ml Fifth layer2.6 ml Sixth layer 3.0 ml Seventh layer 3.5 ml Eighth layer 4.3 ml

While the GRIN lens preform 24 was inserted into an electric furnace ofa carbon heater at a rate of 0.04 mm/s, the preform was formed into theGRIN lens-like optical fiber having 150 μm outer diameter by spinning,and the first GRIN lens-like optical fiber was thereby formed. Therefractive index distribution of the first GRIN lens-like optical fiberthus formed was measured, and as a result, the refractive indexdistribution of the first GRIN lens-like optical fiber was obtained in aprofile of substantially square curved decrease expressed by numericalaperture satisfying NA=0.53 from the center toward the periphery. Here,at the time of spinning at 1900° C. or more, the titanium component inthe center part slightly flies around, and therefore, as shown in table1, the titanium added amount of the eight layers is increased to preventdeteriorating the refractive index.

The second GRIN lens-like optical fiber thus formed was fusion-splicedwith one end of the single mode optical fiber having numerical aperture0.15 by using the discharge fusion splicer. Thereafter, the second GRINlens-like optical fiber and the single mode optical fiber thusfusion-spliced was cut in a 990 μm length which is nearly ¼ as long as azigzag cycle of a light beam propagated through the second GRIN lens.Next, the first GRIN lens-like optical fiber having 0.53 numericalaperture different from that of the second GRIN lens was fusion-splicedwith the second GRIN lens by using the same discharge fusion splicer asdescribed before. Thereafter, the first GRIN lens-like optical fiber andthe second GRIN lens thus fusion-spliced was cut and ground so as to be840 μm in length, and the optical fiber coupling part of a secondembodiment was obtained.

The optical fiber coupling part thus obtained was opposed to thesemiconductor laser having 1330 nm peak oscillation wavelength, 16 mAoperating current, 1.0V operating voltage, horizontally radiation fullangle at half maximum 20°, and vertically radiation full angle at halfmaximum 25°, holding operating distance of 30 μm therebetween, and inthis case, high coupling efficiency with coupling loss of 0.9 dB or lesswas obtained.

Embodiment 3

First, in the same process as the first embodiment, after forming thepreform of the second GRIN lens having numerical aperture satisfyingNA=0.16 was formed, the preform was formed into the second GRINlens-like optical fiber having 150 μm outer diameter by spinning in theelectric furnace of the carbon heater.

Subsequently, tantalum ethoxide was added to and mixed with 1.1 gsilicon tetramethoxide by an amount of eight kinds shown in table 2, and1.3 cc methanol was further added and mixed therein, and the mixturethus obtained was stirred. Thereafter, 0.3 g superfine particulatesilica was mixed therein, and after stirring for one hour, 1.3 ccmethanol and 0.3 cc pure water were mixed and delivered by drops thereinto adjust the sol.

First, the sol of the first layer was contained in the cylindricalpolypropylene vessel having 50 mm inner diameter, and the sol thuscontained was rotated at a speed of 1000 revolutions per one minute for30 minutes to form a cylindrical gel on the inner wall of the vessel.Thereafter, in the same process, sol liquid with different titaniumcomponent from second layer to eighth layer was sequentially containedin the vessel to concentrically laminate a wet gel layer with differentadded amount of titanium of eight kinds on the inner wall of the vessel.While rotating the cylindrical wet gel thus formed, the wet gel wasdried at 60° C. for one week to obtain a dry gel. The dry gel was formedin a cylinder having 25 mm inner diameter, 14 mm outer diameter, and0.04% or less ellipticity. The dry gel thus obtained was subjected totemperature-raise to 800° C. at a rate of 150° C./hr in an oxygenatmosphere, and thereafter, to 1250° C. at a rate of 50° C./hr in ahelium atmosphere, and then subjected to baking to obtain a transparentglass body. TABLE 2 Tantalum added amount First layer   0 g Second layer0.6 g Third layer 0.9 g Fourth layer 1.2 g Fifth layer 1.5 g Sixth layer1.7 g Seventh layer 2.0 g Eighth layer 2.2 g

From the cylindrical glass body, the closed cylindrical GRIN lenspreform was formed in the same way as the embodiment 2, while thepreform was inserted into an electric furnace of a carbon heater at arate of 0.04 mm/s, the preform was formed into the first GRIN lens-likeoptical fiber having 150 μm outer diameter by spinning, and the firstGRIN lens-like optical fiber was thereby formed. The refractive indexdistribution of the first GRIN lens-like optical fiber thus formed wasmeasured, and as a result, it was clarified that the preform of thesecond GRIN lens had a profile of substantially square curved decreaseexpressed by numerical aperture satisfying NA=0.52, from the centertoward the periphery. In a case of tantalum, the tantalum flying aroundwas not checked, as described in the second embodiment.

The second GRIN lens-like optical fiber thus formed was fusion-splicedwith one end of the single mode optical fiber having 0.15 numericalaperture by using the discharge fusion-splicer. Thereafter, the secondGRIN lens-like optical fiber and the single mode optical fiber thusfusion-spliced was cut in a 990 μm length which is nearly ¼ as long as azigzag cycle of a light beam propagated through the second GRIN lens.Next, the first GRIN lens-like optical fiber having numerical aperture0.53 different from that of the second GRIN lens was fusion-spliced withthe second GRIN lens by using the same discharge fusion-splicer.Thereafter, the first GRIN lens-like optical fiber was cut and ground soas to be 840 μm length, and the optical fiber coupling part of a thirdembodiment was obtained.

The optical fiber coupling part thus obtained was opposed to thesemiconductor laser having 1330 nm peak oscillation wavelength, 16 mAoperating current, 1.0V operating voltage, horizontally radiation fullangle at half maximum 20°, and vertically radiation full angle at halfmaximum 25°, holding operating distance of 30 μm therebetween, and inthis case, high coupling efficiency with coupling loss of 0.9 dB or lesswas obtained.

First, in the same process as the first embodiment, after forming thepreform of the second GRIN lens having numerical aperture satisfyingNA=0.16, the preform was formed into the second GRIN lens-like opticalfiber having 150 μm outer diameter by spinning in the electric furnaceof the carbon heater.

Next, silicon tetramethoxide, pure water, and hydrochloric acid weremixed at a molar ratio of 1:5:0.001, the mixture thus obtained wasstirred until hydrolysis was completely ended, and sol liquid wasthereby obtained. Thereafter, superfine particulate silica was mixed inthe sol liquid so that a weight ratio to SiO₂ in the sol becomes 40%,and the mixture was adequately stirred. Thereafter, 0.1 normal ammoniawater was added to and the sol was adjusted. The sol thus adjusted wascontained in the cylindrical polypropylene vessel having 50 mm innerdiameter, and the sol thus contained was rotated at a speed of 1000revolutions per one minute for two hours, to make a cylindrical wet gelon the inner wall of the vessel. The 50 g wet gel was immersed inprocessing liquid 800 ml mixed with isopropanol and ascetone in whichmolecular sieve 3A was added, and the processing liquid was stirred for24 hours. Thereafter, the processing liquid was replaced, and the sameoperation was repeated. Thereafter, solution mixed with 5 g titaniumtetra-n-butoxide and 70 ml ethanol was poured into a cylinder shape andstirred for 5 hours, and the concentration distribution of titanium wasimparted to the cylindrical wet gel. The gel was immersed in acetone,and titanium was fixed in the fine pores of the gel.

While rotating the cylindrical wet gel having the concentrationdistribution of titanium thus formed, the wet gel was dried at 60° C.for one week to obtain a dry gel. The dry gel was contracted and formedinto a cylinder having a 26 mm inner diameter, a 13 mm outer diameter,and 0.04% or less ellipticity. The dry gel thus obtained was subjectedto temperature-raise from room temperature to 800° C. at a rate of 150°C./hr in an oxygen atmosphere, and thereafter to 1250° C. at a rate of50° C./hr in a helium atmosphere, and then subjected to baking to obtaina transparent glass body.

From the cylindrical glass body, in the same way as the second and thirdembodiments, the closed cylindrical GRIN lens preform was formed, andwhile the GRIN lens preform was inserted into an electric furnace of acarbon heater at a rate of 0.04 mm/s, the preform was formed into theGRIN lens-like optical fiber having a 150 μm outer diameter by spinning,and the first GRIN lens-like optical fiber was thereby formed. Therefractive index distribution of the first GRIN lens-like optical fiberthus formed was measured, and as a result, the refractive indexdistribution of the preform of the first GRIN lens was obtained in theprofile of substantially square curved decrease expressed by numericalaperture satisfying NA=0.48 from the center toward the periphery.

The second GRIN lens-like optical fiber thus formed was fusion-splicedwith one end of the single mode optical fiber having numerical aperture0.15 by using the discharge fusion splicer. Thereafter, the second GRINlens-like optical fiber and the single mode optical fiber thus fusionspliced was cut in a 990 μm length which is nearly ¼ as long as a zigzagcycle of a light beam propagated through the second GRIN lens. Next, thefirst GRIN lens-like optical fiber having numerical aperture 0.48different from that of the second GRIN lens was fusion-spliced with thesecond GRIN lens by using the same discharge fusion splicer as describedbefore. Thereafter, the first GRIN lens-like optical fiber and thesecond GRIN lens thus fusion-spliced was cut and ground so as to be 890μm length, and the optical fiber coupling part of a fourth embodimentwas obtained.

The optical fiber coupling part thus obtained was opposed to thesemiconductor laser having 1330 nm peak oscillation wavelength, 16 mAoperating current, 1.0V operating voltage, horizontally radiation fullangle at half maximum 20°, and vertically radiation full angle at halfmaximum 25°, holding operating distance of 30 μm therebetween, and inthis case, high coupling efficiency with coupling loss of 0.9 dB or lesswas obtained.

Only the outer wall of the wet gel in which the titanium concentrationdistribution was fixed, and which is formed in the same way as thefourth embodiment, was immersed in 6 normal hydrochloric acid for 5minutes, and a titanium additive fixed to the periphery of the wet gelwas removed, thereby imparting a steep titanium concentrationdistribution. The wet gel thus obtained was immersed in methanol, and bywashing the hydrochloric acid, the wet gel was dried to obtain a dry gelhaving 26 mm inner diameter and 13 mm outer diameter. The dry gel thusobtained was contained in a tubular furnace, and was subjected totemperature-raise from room temperature to 350° C. at a rate of 10°C./hr, and thereafter to 1200° C., and then subjected to baking toobtain a transparent cylindrical glass body. The glass body thusobtained was spun into 150 μm in the same way as the first embodiment,and the refractive index distribution was measured. As a result, theGRIN lens was obtained, having high numerical aperture 0.55 andrefractive index distribution which was closer to a square curve thanthat of the first embodiment. In the same way as the first to fourthembodiments, when the optical fiber coupling part is formed byspecifying the GRIN lens thus obtained as the first GRIN lens, theoptical fiber coupling part having high coupling efficiency can beobtained.

Further, instead of using the titanium tetra-n-butoxide of fourthembodiment, tantalum propoxide Ta(OC₃H₇)₅ was used to fix theconcentration distribution of tantalum, and then the wet gel was driedand baked. However, the refractive index distribution of the glass thusobtained had the profile of decreasing square curved refractive indexdistribution of numerical aperture satisfying NA=0.52. The GRIN lensthus obtained is specified as the first GRIN lens, and the optical fibercoupling part is formed in the same way as the first to fourthembodiments, and in this case, the optical fiber coupling part havinghigh coupling efficiency can be obtained.

1-10. (canceled)
 11. An optical fiber coupling part comprising: anoptical fiber; and at least one GRIN lens fusion-spliced with an end ofsaid optical fiber, said GRIN lens having a numerical aperture NA thatis larger than a numerical aperture NA_(s) of a light emitting source.12. The optical fiber coupling part according to claim 11, wherein thenumerical aperture NA is 0.43 or more.
 13. The optical fiber couplingpart according to claim 12, wherein the GRIN lens has a coefficient ofthermal expansion expressed by 15×10⁻⁷K⁻¹ or less, and is formed by asol-gel method.
 14. The optical fiber coupling part according to claim11, wherein the GRIN lens has a coefficient of thermal expansionexpressed by 15×10⁻⁷K⁻¹ or less, and is formed by a sol-gel method. 15.The optical fiber coupling part according to claim 11, wherein theoptical fiber comprises a single mode optical fiber.
 16. An opticalfiber coupling part comprising: an optical fiber having numericalaperture NA_(f); a first GRIN lens having numerical aperture NA₁; and asecond GRIN lens having a numerical aperture NA₂, wherein a first end ofsaid second GRIN lens is fusion spliced with an end of said opticalfiber and a second end of said second GRIN lens is fusion spliced withsaid first GRIN lens, wherein numerical aperture NA₁ is larger thannumerical aperture NA₂.
 17. The optical fiber coupling part according toclaim 16, wherein the numerical aperture NA_(f) of the optical fiber,the numerical aperture NA₁ of the first GRIN lens, the numericalaperture NA₂ of the second GRIN lens, and the numerical aperture NAs ofa light emitting source are selected to satisfy the formula expressedby:NA _(f) ≦NA ₂ <NA _(s) ≦NA ₁.
 18. The optical fiber coupling partaccording to claim 17, wherein the numerical aperture NA₁ of said firstGRIN is 0.43 or more.
 19. The optical fiber coupling part according toclaim 17, wherein a length Z₁ of the first GRIN lens satisfies theformula expressed by:Z ₁=(n _(o) *d ₁ /NA ₁)arctan(d ₁/(NA ₁ *L) wherein a refractive indexof glass at a center part of the first GRIN lens is set at n_(o), aradius of the first GRIN lens is set at d₁, and a distance between thelens and the light emitting source is set at L.
 20. The optical fibercoupling part according to claim 17, wherein said first GRIN lens andsaid second GRIN lens have a coefficient of thermal expansion expressedby 15×10⁻⁷K⁻¹ or less, and at least the first GRIN lens is made by asol-gel method.
 21. The optical fiber coupling part according to claim16, wherein the numerical aperture NA₁ of said first GRIN is 0.43 ormore.
 22. The optical fiber coupling part according to claim 21, whereina length Z₁ of the first GRIN lens satisfies the formula expressed by:Z ₁=(n _(o) *d ₁ /NA ₁)arctan(d ₁/(NA ₁ *L) wherein a refractive indexof glass at a center part of the first GRIN lens is set at n_(o), aradius of the first GRIN lens is set at d₁, and a distance between thelens and the light emitting source is set at L.
 23. The optical fibercoupling part according to claim 21, wherein said first GRIN lens andsaid second GRIN lens have a coefficient of thermal expansion expressedby 15×10⁻⁷K⁻¹ or less, and at least the first GRIN lens is made by asol-gel method.
 24. The optical fiber coupling part according to claim16, wherein a length Z₁ of the first GRIN lens satisfies the formulaexpressed by:Z ₁=(n _(o) *d ₁ /NA ₁)arctan(d ₁/(NA ₁ *L) wherein a refractive indexof glass at a center part of the first GRIN lens is set at n_(o), aradius of the first GRIN lens is set at d₁, and a distance between thelens and the light emitting source is set at L.
 25. The optical fibercoupling part according to claim 24, wherein a length Z₂ of said secondGRIN lens is nearly ¼ as long as a zigzag cycle of a light beampropagated through said second GRIN lens or a length that is an oddnumber times the length of ¼ of the zigzag cycle.
 26. The optical fibercoupling part according to claim 25, wherein said first GRIN lens andsaid second GRIN lens have a coefficient of thermal expansion expressedby 15×10⁻⁷K⁻¹ or less, and at least the first GRIN lens is made by asol-gel method.
 27. The optical fiber coupling part according to claim24, wherein said first GRIN lens and said second GRIN lens have acoefficient of thermal expansion expressed by 15×10⁻⁷K⁻¹ or less, and atleast the first GRIN lens is made by a sol-gel method.
 28. The opticalfiber coupling part according to claim 16, wherein said first GRIN lensand said second GRIN lens have a coefficient of thermal expansionexpressed by 15×10⁻⁷K⁻¹ or less, and at least the first GRIN lens ismade by a sol-gel method.
 29. The optical fiber coupling part accordingto claim 16, wherein the optical fiber comprises a single mode opticalfiber.